The present invention concerns a method of purifying lithium hexafluorosphate that allows to purify lithium hexafluorophosphate, useful as, for instance, lithium secondary cell electrolyte, organic synthesis medium or the like, to an extremely high purity.
Lithium hexafluorophosphate is usually synthesized in an insoluble medium such as hydrofluoric anhydride or organic medium; however it reacts with a trace of water existing in the medium, and generates oxyfluoride and lithium fluoride etc., then they are mixed into the product and contaminates the same. Besides, lithium hexafluorophosphate decomposes and equilibrates partially as shown in the following expression, when it is dissolved in hydrofluoric anhydride or organic medium.
LiPF6←xe2x86x92LiF+PF5
This LiF is mixed into the product and becomes impurity.
As lithium hexafluorophosphate has an extremely high moisture absorption ability, even a trace of humidity in the atmosphere provokes hydrolysis to generate fluoric acid portion. In addition, lithium fluoride or lithium oxyfluorophosphate (LiPOxFy) is generated to cause impurity. If lithium hexafluorophosphate containing such impurities is used as lithium cell electrolyte, fluoric acid erodes into electrode material, and lithium fluoride becomes residue as it is insoluble to organic medium and requires filtration and removal, while lithium oxyfluorophosphate accelerates the decomposition of organic medium, and possibly increases inner voltage, lowers cell capacity, or inflames.
Conventionally, these impurities can be removed, by a method wherein lithium hexafluorophosphate is solved in an organic medium and impurities are removed by an ion exchange {exchanged} resin (Japanese Patent Laud-Open No. SHOU 59-87774) or a method for neutralizing in an organic medium (Japanese Patent Laid-Open No. SHOU 59-81870); however, as these methods solve once into an organic medium, increasing the production process and costs.
Moreover, a method for reacting and removing oxyfluorophosphate with fluoric gas (Japanese Patent Publication No. HEI 4-16406, Japanese Patent Publication No. HEI 4-16407) or a method for making hexafluorophosphate contact with a gas containing pentafluorophosphate (Japanese Patent Laid-Open No. HEI 6-29850) or the like involve danger in gas treatment and require special knowledge. Moreover, fluoric gas or phosphor pentafluoride to be used are required to be refined to a high purity, increasing cost and affecting considerably the product price.
Therefore, none of conventional method was not satisfactory.
It is an object of the present invention to provide a method of purifying lithium hexafluorosphate that allows to obtain high purify lithium hexafluorophosphate, by converting hydrogen fluoride, oxyfluoride, lithium fluoride or other impurities, generated in the course of production, handling or conservation of lithium hexafluorophosphate and mixed with the product, into lithium hexafluorophosphate.
The method of purifying lithium hexafluorosphate of the present invention is characterized by that the purification is performed by introducing phosphoric chloride and hydrogen fluoride of the quantity necessary for producing phosphor pentafluoride of the quantity more than necessary for converting rude lithium hexafluorophosphate and oxyfluoride in the raw material into lithium hexafluorophosphate, in the reaction system with brut lithium hexafluorophosphate and reacting them.
In the present invention, impurities such as hydrogen fluoride, oxyfluoride, lithium fluoride contained in lithium hexafluorophosphate are converted into lithium hexafluorophosphate by adding phosphoric chloride for the purification. At this moment, phosphoric chloride and hydrogen fluoride of equal or superior to the equivalent amount are introduced for reacting them.
At this moment, phosphoric chloride reacts with hydrogen fluoride to generate phosphor pentafluoride:
xe2x80x83PCl5+5HFxe2x86x92PF5+5HClxe2x80x83xe2x80x83(1)
The generated phosphor pentafluoride reacts respectively with oxyfluoride and lithium fluoride as follows to be converted into lithium hexafluorophosphate and purified.
LiPOxFy+PF5xe2x86x92LiPF6+POxFyxe2x88x921xe2x80x83xe2x80x83(2)
LiF+PF5xe2x86x92LiPF6xe2x80x83xe2x80x83(3)
The reaction shown by the formula (1) develops quantitatively at the room temperature and normal pressures. By-product hydrochloric gas is not involved into the reaction and discharged from the system without depositing to the product or remaining, because of high vapor pressure.
The ratio of phosphoric chloride to hydrogen fluoride is desirable that hydrogen fluoride be equal or superior to chemical equivalent amount; however, even if phosphoric chloride is in-excess and remains non-reacted, it can be removed by heating at an temperature equal or superior to 100xc2x0 C. because its sublimation temperature is 100xc2x0 C.
The reaction of phosphor pentafluoride and oxyfluoride shown in the formula (2) and the reaction of phosphor pentafluoride and lithium fluoride shown in the formula (3) depend on the temperature. The reaction develops easily if the temperature is high. However, as the temperature attains 250xc2x0 C. or more, the counter-reaction as shown below will occur, increasing the content of lithium fluoride as impurity. FIG. 1 shows an example of differential thermal analysis of lithium hexafluorophosphate.
xe2x80x83LiF+PF5←xe2x86x92LiPF6 greater than 250xc2x0 C.xe2x80x83xe2x80x83(4)
Though the reaction of phosphor pentafluoride with oxyfluoride, lithium fluoride proceeds sufficiently around the normal pressure, it is desirable that the reaction develops under the pressure. If the reaction system is put under the negative pressure, not only the reaction of phosphor pentafluoride with oxyfluoride, lithium fluoride proceeds hardly, but, on the contrary, lithium hexafluorophosphate decomposed increasing impurities. This is shown, in FIG. 2, by the relationship between free acid and insoluble residue by the inner pressure of the reaction system. Additionally, the time necessary for this generation reaction depends on conditions such as temperature or pressure, and the content of hydrogen fluoride, oxyfluoride and lithium fluoride in lithium hexafluorophosphate used for the generation, however several hours to a whole day and night are sufficient. When the purification reaction is completed, through dry nitrogen gas, remaining hydrogen fluoride, phosphor pentafluoride, hydrochloride, and non-reacted phosphoric chloride gas are purged completely.